1. Field of the Invention
This invention relates to the electronic and electrical arts and more particularly, to specific and novel type of conductive materials obtained from heat-resistant organic polymeric materials by thermal or pyrolytic treatment. It also relates to conductive compositions using such a specific type of conductive pyrolytic materials in the form of powders.
2. Description of the Prior Art
In recent years, attempts have been made to actively develop sensors or heaters using organic materials which have been imparted with electrical conductivity and showing a specific performance which is not experienced in those using known metallic conductors. In 1964, S. D. Bruck of IBM Corp. discovered that when thermally decomposed or pyrolyzed at temperatures higher than 800.degree. C., a polyimide film (Kapton H film of I. E. Du Pont de Nemours) is converted to an electrical conductor having a specific resistance of 5.times.10.sup.-2 .OMEGA.cm. The specific resistance of polyimide film is usually at a level of 10.sup.-18 .OMEGA.cm at a normal temperature and thus the film varies in resistance in the order of 10.sup.20 by the pyrolysis. In one sense, this pyrolytic technique is considered as excellent to impart electrical conductivity to the polymer. However, the technique is not always applicable to all polymer materials. In fact, most polymeric materials are not converted into materials of high conductivity when subjected to the pyrolysis. Further, the conductors obtained by the pyrolysis have a common disadvantage in that they are brittle and show much reduced flexibility, coupled with another disadvantage that a maximum value of conductivity is about 200.OMEGA..sup.-1 cm.sup.-1 which is unsatisfactory as a conductor.
Fundamental requirements for polymeric materials which are capable of being converted into materials of high conductivity by the pyrolytic technique are considered as follows: (1) The pyrolysis is stopped at a certain stage; (2) A re-combination reaction proceeds as the pyrolysis proceeds; and (3) There are present unsaturation bondings in sufficient amounts in the re-combined product. However, it is not known at present what type of polymeric material or molecular structure can satisfy the above requirements. In order to satisfy the requirements, it seems essential that the melting point of the polymeric material be higher than its decomposing temperature. This is because a material whose melting point is lower than its decomposing temperature will be melted and evaporated on heating at the decomposing temperature prior to occurrence of the pyrolytic reaction. In view of the above, widely employed polymeric materials such as polyester, polystyrene, polyvinyl chloride, polyethylene, polypropylene and the like can not be used for the purpose. Accordingly, the pyrolysis technique may be applicable to heat-resistant polymers such as polyimide, polybenzimidazole, polydiphenyl ether, polyparaphenylene and the like. However, the condition where the melting point is higher than the decomposing temperature is a necessary one but not a satisfactory one. For instance, in the case of polyparaphenylene, its pyrolytic reaction does not stop at a stage and continues to proceed until it is decomposed.
Organic conductors obtained by pyrolysis can be classified as low dimensional conductors. A low dimensional conductor means a material which shows high electrical conductivity only in a linear direction (i.e. one dimension) or in a plane direction (two dimensions) which is different from known three-dimensional conductors as is discussed in "Chemistry and Industry", April, 1979, pp. 221-240. That is, it is assumed that organic conductors of pyrolytic polymers in which starting polymers undergo a condensation reaction by pyrolysis have a plane or two-dimensional structure. As described hereinbefore, it is reported that the Kapton H film (polyimide) of I. E. Du Pont de Nemours is converted into a conductor when pyrolyzed in vacuo ("Polymer" vol. 5, page 435, 1964). Further, it is also reported by a Russian group in 1963 that polyacronitrile is converted by pyrolysis into a material having a specific resistance of 5.times.10.sup.-1 .OMEGA.cm (A.V. Airapetjanc et al, Dokl. Akad, Nauk SSR Vol. 148, p. 605, 1963). These pyrolytic polymers are advantageous in their high resistance to heat but are considered to be inferior to carbon black or graphite when applied as a material of low resistance. Further, they are also considerably poorer in film-forming properties and flexibility as compared with ordinary polymers and can not stand use even when supported on a base material.